1. Field of the Invention
The present invention generally relates to a projection exposure method and apparatus for producing, for example, a semiconductor device or a liquid crystal display device, and, more particularly, to a projection exposure method and apparatus, in which the posture of a substrate, such as a mask or a semiconductor wafer, used for producing a semiconductor device, a liquid crystal display device, or the like, is controlled.
2. Description of the Related Art
Conventionally, in an exposure apparatus of the aforementioned type, a surface of a wafer, coated with a photosensitive material, is brought into alignment with an optimal image-forming plane of a projection optical system by determining the position of the wafer with a sensor, which detects displacement in the focusing direction or tilting of the wafer, and by driving a mechanism, based on the detected displacement or tilting. In this case, determining the focusing direction after positioning an X-Y stage, which drives the wafer in a planar direction significantly reduces productivity. On the other hand, when, in order to increase productivity, the focusing direction is determined before the X-Y stage stops moving, the precision with which the focusing direction is determined is reduced, so that the wafer surface cannot be brought into alignment with, for example, an optimal image forming plane.
In the conventional projection exposure apparatus described above, productivity is ordinarily increased by determining the vertical (Z) direction before the stage stops vibrating in,the focusing direction. The correct vertical direction, however, cannot be accurately determined, as a result of the stage vibration. It is possible to wait for the stage to stop vibrating before determining the vertical direction, but determining the vertical direction in this way takes additional time, since one must wait for the stage to stop vibrating. As a result, productivity is reduced. Accordingly, when steps are taken to measure the wafer location more precisely, more time is required to measure the wafer location, whereas when steps are taken to measure the wafer location in a shorter time, the wafer location is measured less precisely.
Accordingly, in order to overcome the above-described conflicting problems, it is an object of the present invention to provide a method that allows productivity to be increased, without reducing precision and increasing measuring time.
In a first aspect, the present invention provides a projection exposure method that includes steps of moving a substrate, which is placed on a stage that is movable in a direction of an optical axis of a projection optical system and in a direction perpendicular thereto, in the direction perpendicular to the optical axis of the projection optical system in order to successively move a plurality of exposure areas on the substrate to respective, predetermined exposure locations, determining and storing a correction value obtained after moving the exposure areas on the substrate such that the surface of the substrate is aligned with an optimal image-forming location, measuring the amount of displacement of each of the exposure areas of the substrate surface from the optimal image forming location of the projection optical system, moving the exposure areas of the substrate surface, based on the measured values, for alignment with the optimal image forming location, correcting the measured displacements based on the determined correction value, and exposing the substrate surface.
The step-of-measuring the amount of displacement of the exposure areas from the optical image forming location and the step of moving the exposure areas of the substrate surface to the optimal image forming location can be at least partly performed concurrently with a step of moving the substrate in the direction perpendicular to the optical axis of the projection optical system.
The method can further include setting a timing of starting the step of measuring the amount of displacement of the exposure areas with reference to the amount of time that has elapsed since the stage has started moving in the direction perpendicular to the optical axis of the projection optical system.
The step of measuring the amount of displacement of the exposure areas can be started when the amount of displacement from a target location becomes at most a predetermined value during movement of the stage in the direction perpendicular to the optical axis of the projection optical system.
In another aspect, the present invention provides a projection exposure apparatus that includes a projection optical system for projecting a pattern formed on a base onto a photosensitive substrate, a stage, movable in a direction of an optical axis of said projection optical system and in a direction perpendicular thereto, for holding the photosensitive substrate, a sensor for measuring displacement in a focusing location, which occurs when the surface of the photosensitive substrate on the stage is displaced from a predetermined location in the optical axis direction of the projection optical system, means for adjusting the location of said stage in the optical axis direction of said projection optical system in order to align the photosensitive substrate on said stage with the predetermined location in the optical axis direction of said projection optical system, based on the measured value obtained by said sensor, means for generating a relationship between the location of said stage in the direction perpendicular to the optical axis of said projection optical system and a correction value indicating the displacement in the focusing location obtained after said adjustment means adjusts the location of said stage, and for storing the generated relationship, determining means for determining the amount of displacement in the focusing location of the photosensitive substrate based on the displacement in the focusing location measured by said sensor, after the photosensitive substrate is moved, during exposure, to a predetermined location in a plane extending in the direction perpendicular to the optical axis of said projection optical system, means for correcting the determined amount of displacement, based on the correction value stored in said storing means, and means for moving said stage in the optical axis direction of said projection optical system in order to align the photosensitive substrate with the predetermined location, based on the correction made by said correction means.
The apparatus can further comprise means for causing a correction value, obtained by a subsequent measurement, and a location from the stage, obtained when the initial correction value is equal to the correction value obtained by the subsequent measurement to be stored in the storing means, wherein the initial measurement is performed, after the photosensitive substrate is moved to the predetermined location on the plane extending perpendicular to the optical axis of the projection optical system, to measure the amount of displacement in the focusing location of the photosensitive substrate using the sensor, in order to move the photosensitive substrate vertically to the predetermined location, based on the initial measurement, and wherein the subsequent measurement is performed, using the sensor to measure the amount of displacement in the focusing location of the photosensitive substrate, which amount of displacement measured in the subsequent measurement is the correction value.
The initial measurement can be started, along with movement of the stage, either after a certain amount of time has elapsed from a time when the photosensitive substrate has started moving to the predetermined location on the plane extending perpendicular to the optical axis of the projection optical system or when the displacement from the predetermined target location becomes at most a predetermined value.
The initial measurement for generating data to be stored in the storing means can be performed without exposing the photosensitive substrate during the data measurements, and the photosensitive substrate can thereafter be exposed during subsequent measurements.
The correction value can indicate the displacement in the focusing location in the direction of the optical axis of the projection optical system and in a direction of axial rotation perpendicular to the direction of the optical axis of the projection optical system.
The predetermined location in the optical axis direction of the projection optical system can correspond to one of an optimal image forming plane of the projection optical system and an image forming plane of an alignment measuring system used for bringing the photosensitive substrate within a certain target plane.
The determining means can determine the amount of displacement in the focusing location of a substrate other than the photosensitive substrate used for exposure.
The apparatus can further comprise means for positioning the photosensitive substrate, being moved in the optical axis direction of the projection optical system, to a location spaced by a certain amount from the optimal forming plane.
The means for moving the stage in the optical axis direction of the projection optical system is capable of returning the photosensitive substrate, being tilted, to a predetermined orientation.
The correcting means can comprise a control section which uses one of a correcting method corresponding to the size and type of a pattern formed on the base and data stored in the storing means.
The control section can comprise numerical value processing means for performing weighting of the correction value and the position of the stage previously stored in the storing means, and for deleting any data values falling outside one of a predetermined movement average and a predetermined permissible range.
The control section sets intervals between subsequent measurements based on an allowable amount of displacement in the focusing location, an exposure amount and a processing time of the photosensitive substrate, with the allowable a mount of displacement, exposure amount and processing time depending on the pattern to be formed.
In yet another aspect, the present invention provides a semiconductor device produced using either the projection exposure method of the present invention or the projection exposure apparatus of the present invention.